Linear actuators are used to move an object along a straight line, either between two end points or to a defined position. Actuators may be air or hydraulic driven using pressure or they may be driven by electricity. Air or hydraulic driven actuators are cheap and simple in design. They are also easy to control, but they require an air or hydraulic supply which is relatively expensive, especially for small systems.
The control of a pneumatic actuator is done with a system comprising controlled valves and sensors, normally in the form of limit switches. The control system may be completely pneumatic, i.e. comprising air controlled valves and sensors opening or closing an air supply. Such systems may be used in small and simple applications, when only a few actuators and sensors are used. In larger applications, the system is normally electro-pneumatic having electrically controlled valves and electronic position sensors, e.g. proximity sensors or limit switches. The position sensors give a signal when the actuator has reached a predefined position, and may be positioned on the actuator such that they will detect the end positions of the actuator itself, or they may be positioned externally from the actuator, detecting end positions of e.g. a work piece. The electrically controlled valves will open selectively to feed air to the piston so that the actuator will extend or retract.
There are different types of control systems used in the industry. In larger systems, the different actuators and sensors will be connected to a bus-system of some kind, e.g. fieldbus, profibus, CANopen or DeviceNet. These bus systems require a bus interface for each connected node. A node may comprise a bus controlled I/O-interface of some kind. In less advanced systems, the control system may be a PLC (Programmable Logic Controller) system, in which one or more PLC units are connected to the actuators and sensors via input and output ports. In this case, an electric linear actuator and a separate control unit with a PLC interface for the connection to the control system will be used.
A simple control system for controlling an electro-pneumatic linear actuator will use four control signals, two control output signals and two sensor input signals. The output signals will be one signal for travelling up/extend and one signal for travelling down/retract. When the signal is e.g. high, the actuator will travel and when the signal is low, the actuator will rest. The input signals detects when the actuator has reached an end stop, e.g. the maximum/extended and minimum/retracted end stops. One sensor may e.g. give a high output when that sensor has detected the maximum/extended position.
Electrically driven linear actuators normally incorporate a rotating motor and some kind of transmission means to convert the relatively high-speed rotating motor to a low speed linear motion. This transmission means may incorporate a gear box and/or a screw shaft. One common type of linear actuator incorporates a screw shaft with a nut running thereon. The screw shaft extends over the full length of the actuator and sets the operating length of the actuator. Since the nut is held in a non-rotatable state, the nut will be displaced when the screw shaft is rotated by the motor. The nut may incorporate rolling elements, such as balls or rollers, between the screw shaft and the nut. This will allow for a high efficiency actuator with high load transfer and long life. The nut may also engage directly with the screw shaft, i.e. a sliding screw design. In this case, the nut is preferably made of a plastic material.
Both pneumatic and hydraulic linear actuators are widely used in the industry, partly for historic reasons. Earlier, especially air was considered as a cheap, safe and user-friendly energy source compared with electricity. This is not the case anymore. Thus, there are some advantages in replacing electrically controlled pneumatic linear actuators with electric linear actuators. However, since electric linear actuators have different mechanical properties and also have a different electric interface, the replacement is often only possible or reasonable to carry out when a redesign of the system is realised.
There is thus room for an improved electric linear actuator.